In today's complex electronic systems, one of the primary objectives is accuracy of performance as well as speed of performance. Often times, the accuracy of performance of a given electronic application is directly related to the stability of various parameters of the source of power for that system. One such parameter which can significantly affect accuracy of performance of an electronic system is the stability of the frequency of the input power to that system. It is desirable that there is a capability to continuously monitor input frequency and to either indicate an exceeding of limits of input frequency by such means as meters or alarms or by employing the indicator signal as a feedback signal to a device to automatically adjust the input frequency back toward the desired value. Such monitoring of input frequency is readily accomplished by use of a frequency transducer. A frequency transducer converts AC input frequency into a DC signal which is zero at a given frequency (generally at that frequency which is desired on the input line) and varies from zero proportionally with the variance of the input frequency from the desired frequency. The direction of variance, positive or nagative, of the DC output indicates whether the frequency variance is toward a higher frequency or toward a lower frequency.
Most frequency transducers in the art employ some sort of timer circuit as part of the transducing scheme. By and large, these time circuits employ an RC (resistive-capacitive) circuit. One source of inaccuracy with such RC circuits is the shunting effect across the capacitor because of leakage caused by increased humidity, circuit board contamination and the like. Such a shunting effect can significantly alter the time constant of the RC circuit and thereby significantly adversely affect the accuracy of the output of the transducer. There is little one can do to alter the magnitude of the change in time constant due to the shunting effect of the capacitor under conditions of increased humidity, however, the present invention significantly lessens the impact of that shunting effect. The shunting effect across the capacitor is nearly constant for a given temperature-humidity condition. The present invention recognizes that fact and is designed to lessen the impact of that inaccuracy by providing a higher charging current for the capacitor in the RC circuit thereby rendering that constant inaccuracy a smaller percentage of the charging current so that the time constant is effectively more stable and the accuracy of the transducer is thereby enhanced.
It is, therefore, an object of this invention to provide a frequency transducer of significantly greater accuracy by ensuring that the effects upon the RC circuit and the timer portion of the transducer occasioned by changes in temperature and humidity are a small percentage of the charging current applied to that RC circuit. The inaccuracies occasioned by the shunting effect across capacitor in the RC circuit because of temperature-humidity conditions is, therefore, recognized as a design criteria and the design of the present invention recognizes that elimination of that error is impractical yet aims at minimizing its effects upon the accuracy of the output of the invention.
Another problem which has been encountered in the design of frequency transducers is the meeting of industry established standards of a peak-to-peak variance of AC ripple signal superimposed upon the DC output signal which is less than 1% of the full scale DC signal output and a less than 400 millisecond response time to changes in frequency of the input from the power line monitored.
It is, therefore, another objective of this invention to provide a frequency transducer which will more easily and more consistently meet the above recited industry standards.
Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings illustrating the preferred embodiment of the invention.